Taiji 9 Levels and Theory

Could write a book on this but I think you've covered the main points pretty well.

This is a big part of all sorts of training. Was doing some yoga stuff involving doing lunge today where the idea was to "hold the ball" behind the front knee. The point was to avoid any chance of shear stress in the joints by building an arch in the back of it...

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Consider Phlebas, who was once handsome and tall as you
High mountains are a feeling
I don't need to sell my soul, he's already in me

Thats exactly the idea, the ball holds the posture and keeps joint stress out of the equation. To continue on with the theory there are other balls in play as well. In Yang style there is another on the chest keeping the hands round and filled with peng. This way in something like horses mane or ward off the movement into the opponent compresses the ball which then creates potential energy to be transferred into the opponent.....

More to come

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"Zen is like riding a Tiger,
Later you become the Tiger."

8-Fist, I agree with all of the above,...plus' I feel/found that you can't/won't have bong/peng WITHOUT lieh, or Jee,or any of the other energies. All connected in some fashion,...like the balence/harmony principle of Yin he' Yang,(Yin 'in-harmony' with Yang). Also,...try holding the floor with your toes. As the rhyme goes; "don't buckle the knuckle',...but, softly press the toes down into the floor as the idea of a monkey holding onto the branch. Use your toes as you would use your fingers,(to hold on'), and you increase your balance/support by 25 to 30%! You can feel ALL of the muscle-groups from the foot/ankle connection up to aprx.' mid-Quads'.! Then, it relates to all of the Tai-Ji energies through the legs/hip connection as well as the arms/shoulder. I found this through doing the 'Split' posture in Sun-style tai-Ji,(it is like the San-Ti Shr posture of Hsing-Yi). Pretty interesting,..and all of my Senior-Citizen students say that it helps when they are going up/down the stairs by holding-on' with their toes.